In May 2013, a stubborn ice jam on the Yukon River sent floodwater spilling over its banks into the small Alaskan town of Galena. An ice jam is an accumulation of broken river ice caught in a narrow channel.

A sharp bend in the river about 18 miles (29 kilometers) from Galena triggered the jam by preventing a large sheet of winter ice from flowing downstream. The blockage began on May 25, and ice and floodwater stretched more than 20 miles (32 kilometers) from the choke point by May 28. Galena residents saw waters surge more than 15 feet (5 meters) in the span of one night.

Viewed from above, the flood transformed the landscape. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured the top image on May 28, 2013. The lower image shows the condition of the river one year earlier. Both images use a combination of visible and infrared light that make it easier to distinguish between water and land. River water appears navy blue; ice appears teal; and vegetation is bright green. Clouds are pale blue-green and cast shadows.

Ice backups and flooding are common on the Yukon River (particularly near Bishop Rock), though not usually this severe. Ice jams can occur at any time of the year, but they are especially common when warming spring temperatures cause rivers to shed their ice. In March 2009, an ice jam caused severe flooding on the Yukon River.

Most of the town’s 400 residents had evacuated by May 28, according to media reports. During the worst of the flooding, the town lost both water and electricity. Most of the structures in the town saw significant flooding; many were swamped by as much as 7 feet (2 meters) of water. There is concern that communities downriver of Galena will also face major flooding when the ice jam breaks up.